For anyone with genuine interest or just morbid fascination, I just posted a story on my boat blog about replacing thru-hulls on my Gulf 32. I tried to post a lot of pictures to clarify the process and outcomes, and let me just say it was a major PIA, but completely worthwhile. They were glassed in, flush head thru-hulls, just to make it more fun!!!:laugher

And hopefully Maine Sail will have some pre-made FG blanks available soon.

bwindrope

04-25-2011 03:20 PM

Maine Sail has great information, including the use of fiberglass blocks, and I do not doubt their effectiveness. However, there are trade offs. Making or buying the fiberglass was far more expensive than the 3/4" plywood. As for rigidity and structure, the plywood is equal in useful strength to the fiberglass, meaning, they are both absolutely solid and rigid for the purpose. And finally, as for longevity and strength, I have a boat that is 23 years old with untreated plywood in various places, including the old 1/4 inch puny backing blocks, and none of it showed any signs of significant decay.

Fully encasing the new 3/4 inch backing blocks with epoxy will provide them with all the longevity I would ever need or expect, and in 20 years time I will likely want to replace them and the thru-hulls again anyway. Another reason for using 4200 and not 5200.

So I would never argue against using fiberglass, at all, it is obviously great, but I believe a solid epoxy covered plywood block will fill all the same purposes except immortality, and I don't expect that anyway.

mitiempo

04-25-2011 03:38 PM

If properly epoxied and epoxied to the hull plywood is probably a fair substitute for fiberglass. Fiberglass is easy to make, can be purchased, or can be obtained as offcuts/waste from a builder.
My job lets me see many boats' bilges and I can't recall wood backing blocks that were not soft and punky with age.

Goblinfog

04-25-2011 04:01 PM

I'm in the mist of redoing mine, but I'm using Densetec Marine Board instead of wood or fiberglass backing. Does anyone see a problem with 5200 and compatability with the marine board? Hope this doesn't bother the thread.

mitiempo

04-25-2011 04:10 PM

My feeling is that the backer should be permanently attached to the hull. Easy to do with fiberglass or plywood using thickened epoxy.
I'm not sure about the Densetec but I don't think many sealants will stick to it and I'm certain epoxy will not.

casey1999

04-25-2011 04:38 PM

Quote:

Originally Posted by treilley
(Post 723956)

And hopefully Maine Sail will have some pre-made FG blanks available soon.

You answered my question- that would be great. I have barely enough time to work on my boat, making back plates would eat up all my time.

mitiempo

04-25-2011 04:38 PM

McMaster Carr sells fiberglass panels in many sizes and grades - up to G10 which is epoxy. It is expensive but you don't need much for a few through hulls.

As I said earlier, builders may have offcuts.

eherlihy

04-25-2011 06:09 PM

Quote:

Originally Posted by bwindrope
(Post 723959)

As for rigidity and structure, the plywood is equal in useful strength to the fiberglass, meaning, they are both absolutely solid and rigid for the purpose. And finally, as for longevity and strength, I have a boat that is 23 years old with untreated plywood in various places, including the old 1/4 inch puny backing blocks, and none of it showed any signs of significant decay.

I removed it because I noticed several small cracks in the bottom paint around the fitting. After I removed it I saw evidence that this through hull had been replaced at least twice (Black unknown sealant, which had been covered with White 5200). I presume that the reason for the earlier replacement was galvanic corrosion, and explained why the bonding wire (visible on the left) was not attached to this fitting. I also found that the genius that installed the latest through hull, put the NPS thread of the through hull into the NPT thread of the seacock...:rolleyes:

The ½" marine plywood backing plate, however, had been mushed down by the nut and allowing the through hull fitting to wobble slightly, thus causing the little cracks. While the plywood had not decayed, it did not support the compressive load that it was subjected to from the through hull fitting and the flange nut. It was definitely weaker than what I used to address the problem. Had I left this, I believe that I would have soon had a leak around the through hull, through the plywood. I believe that it would also have invited osmotic blistering around the through hull.

Because it was relatively new, I re-used the seacock, and the through hull, but added a flange adapter, which adapts from NPS to NPT.

I installed a new backing block made from 3/8" thick G10 (GP03) fiberglass cut into a 6" circle. You can buy a 36" x 6" sheet ready made for less than $25 here. You could make 6 of them from one sheet, and your cost/per would be $5... I could have purchased the same material in ½" thickness for less than $50, but a friend gave me the 3/8" scrap that he had for free!:)

Because 3/8" is thinner than ½", I had to cut some of the threads off of the through hull. Because through hulls use NPS thread, you can easily do this. Just remember to put the flange nut on before you cut. Use the flange nut as a guide while cutting, and to help clean the threads after the cut is complete.

I used black 4200 to seal around the flange fitting, and you can almost see that the bonding wire has been reattached.